In the universe, plasma is the most important state of substance, which is also known as the fourth state of substance, i.e., the ionized “gas.” It exhibits a highly excited and unstable state. The plasma includes ions (having charges of different polarity, i.e. positive or negative), electrons, atoms, and molecules. With the improvement of people's understanding of plasma, plasma technology has been developed rapidly. At present, plasma technology is widely used in the fields of physics, environment, semiconductor, etc., such as plasma smelting, plasma spraying, plasma welding, plasma etching, plasma oxidation, and the like.
The most common method for generating plasma is the gas discharge method. The main excitation methods include: DC discharge, AC discharge, RF discharge, laser excitation and microwave excitation. The microwave plasma is capable of converting microwave energy into internal energy of gas molecules, so that the gas is excited and ionized to form plasma. Compared with other plasmas formed by discharge, microwave plasma has many advantages including: (1) microwave plasma can avoid electrode contamination due to its high-frequency discharge without any electrode; (2) microwave plasma has higher electron temperature, higher electron density and wider emission spectrum; (3) the waveguide can be used to isolate the discharge region from the process region.